Brucellosis is a bacterial infection that causes waves of fever, deep joint pain, and fatigue, and without treatment it can settle into nearly any organ system in the body, including the heart, spine, liver, and brain. Most people catch it by consuming unpasteurized milk or cheese from infected goats, sheep, or cattle, or through direct contact with infected animals. The incubation period ranges from one week to two months, though symptoms typically appear within two to four weeks of exposure.
How It Gets Into the Body and Stays There
What makes Brucella bacteria unusually dangerous is their ability to survive inside the very immune cells sent to destroy them. After entering the body through the gut, lungs, or broken skin, the bacteria are swallowed by immune cells called macrophages. Most bacteria would be killed at this point. Brucella instead hijacks the internal machinery of the cell, redirecting its own compartment into a protected pocket derived from the cell’s protein-building structures. From inside this pocket, the bacteria multiply safely, shielded from both the immune system and many antibiotics.
The bacteria also actively prevent infected cells from self-destructing, a defense mechanism the body normally uses to eliminate compromised cells. By keeping host cells alive, Brucella maintains a stable home. When it’s ready to spread, the bacteria use a process related to the cell’s recycling system to escape and infect new cells. This ability to live and reproduce inside immune cells is the central reason brucellosis is so difficult to clear and so prone to becoming chronic.
Acute Symptoms
The earliest stage of brucellosis often feels like a severe flu that won’t resolve. The hallmark symptoms are fever (often rising and falling in waves), headache, muscle pain, joint aches, and profound fatigue. Many people also experience loss of appetite and weight loss. The fever pattern can be unpredictable, spiking in the afternoon or evening and dropping overnight, which historically earned brucellosis the name “undulant fever.”
Patients with uncomplicated acute brucellosis may recover in two to three weeks, sometimes even without treatment. But that apparent recovery is misleading. Without antibiotics, the bacteria often persist quietly in tissues, setting the stage for chronic disease that can emerge weeks, months, or even years later.
What Happens When It Becomes Chronic
Untreated or inadequately treated brucellosis can become a long-term infection that damages specific organs. The chronic form brings recurring fevers, persistent fatigue, depression, and complications that vary depending on where the bacteria have settled. The most commonly affected systems are the joints and spine, the heart, the liver and spleen, and the central nervous system.
Joints and Spine
Joint inflammation is the most frequent complication. It can appear as general arthritis, but brucellosis has a particular tendency to target the spine (a condition called spondylitis) and the joints where the spine meets the pelvis (sacroiliitis). These cause deep, persistent back and hip pain that worsens over time and can lead to lasting mobility problems if not treated aggressively.
Heart
Endocarditis, an infection of the inner lining of the heart, occurs in roughly 1.3% of brucellosis cases. That number sounds small, but it is the deadliest form of the disease. A systematic review of brucella endocarditis cases found that about 12% of affected patients died and another 11% experienced serious non-fatal complications including relapse or rehospitalization. Left untreated, the infection can destroy heart valves entirely.
Liver and Spleen
The liver and spleen, both heavily involved in filtering blood and managing immune responses, frequently become enlarged and inflamed during brucellosis. This can cause abdominal pain, abnormal liver function, and in some cases abscess formation within these organs.
Brain and Nervous System
Brucella can cross into the central nervous system, causing meningitis (inflammation of the membranes surrounding the brain). Neurobrucellosis can produce severe headaches, confusion, and neurological deficits. It is less common than joint or liver involvement but represents one of the more serious outcomes.
Reproductive System
Brucellosis can affect the reproductive system in both men and women. In men, it can cause orchitis, a painful swelling of the testicles. In women, the infection has been associated with pregnancy complications. While brucellosis is widely known for causing reproductive problems in animals, the effects in humans are generally less dramatic but still clinically significant.
How Brucellosis Is Diagnosed
Diagnosis can be tricky because the symptoms overlap with many other infections. The definitive way to confirm brucellosis is to grow the bacteria from a blood sample or tissue biopsy. This takes time and requires specialized lab handling, since Brucella is a biosafety concern.
Blood tests that measure antibody levels are the most commonly used screening tool. An antibody titer above 1:160, combined with matching symptoms, is considered highly suggestive of infection. Ideally, two blood samples are drawn: one within the first week of symptoms and another two to four weeks later, to see whether antibody levels are rising. A single sample can still support a probable diagnosis when paired with the right clinical picture. Some strains, however, don’t show up on standard antibody tests and require culture confirmation instead.
PCR testing, which detects bacterial DNA directly, is increasingly used and can also help monitor whether treatment is working or whether a relapse has occurred.
Treatment and Recovery
Brucellosis requires a prolonged course of antibiotics, typically a combination of two drugs taken for at least six weeks. The reason for the long treatment and dual-drug approach traces back to the bacteria’s intracellular hiding strategy: because Brucella lives inside cells, antibiotics need sustained exposure to reach and kill it. Shorter or single-drug courses have unacceptably high failure rates.
Even with proper treatment, between 5% and 15% of patients relapse. Relapses usually happen within the first year, which is why follow-up blood tests are recommended for at least 12 months after treatment ends. When relapse occurs, it typically requires another full course of antibiotics.
For uncomplicated cases caught early, most people recover fully. The chronic complications, particularly arthritis, endocarditis, chronic fatigue, and depression, are largely preventable with prompt diagnosis and complete treatment. The longer the infection goes unrecognized, the higher the risk of lasting damage.
Who Is Most at Risk
Brucellosis is most common in parts of the Middle East, Mediterranean, Central Asia, and Latin America, where raw dairy consumption is more prevalent and livestock vaccination programs may be limited. People at highest risk include farmers, veterinarians, slaughterhouse workers, and laboratory personnel who handle Brucella cultures. Travelers who consume unpasteurized cheese or milk in endemic regions also face meaningful exposure risk. The infection is not typically spread from person to person.